This invention relates to image intensifier tubes of the type used in night vision viewing systems and, more particularly, to an image intensifier tube with reduced veiling glare and a method of making the same.
Image intensifier tubes multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing extraterrestial bodies; and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Modern image intensifier tubes utilize a microchannel plate (referred to as an MCP) which is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron multiplier. When the microchannel plate is placed in the plane of an electron image in an intensifier tube, one can achieve a gain of up to several thousand and extremely high resolution. Since each channel in a microchannel plate operates nearly independently of all the others, a bright point source of light will saturate a few channels but will not spread out over adjacent areas. This characteristic of "local saturation" makes these tubes more immune to blooming at bright areas. However, these tubes suffer from a problem known as "veiling glare". Veiling glare is the result of scattered light falling on the light input or focal surface of the tube. In the image intensifier it results in a loss of contrast by filling in the darker portions of the image and decreasing the visibility of small or low contrast objects. In fact, in extreme cases it can cause a complete loss of picture information over a substantial part of the field of view.
Veiling glare is due primarily to off-axis light which is reflected into the inside of the tube and is intensified to appear in the field of view as unwanted images. The sources of veiling glare emanate from bright light rays which are outside the normal field of view; and hence, light rays which are at angles off the axis of view. The light rays emanating from sources outside normal field of view are reflected by the tube and cause the unwanted veiling glare.
There have been various attempts to eliminate or reduce the veiling glare by adding material to the tube which absorbs off-axis light and prevents it from being reflected to the inside of the tube. For example, black rings have been formed on the surface reflecting the off-axis light. These rings have been retained in place by sealing a glass ring to the surface or by fusing a glass ring to the surface to sandwich the ring in between the reflecting surface and the glass ring. This has been difficult to do and is very expensive. Another technique has been to etch a groove between the light input surface and the reflecting surface and to fill the groove with light absorbing material. This, too, has been difficult to do and is also very expensive.
U.S. Pat. No. 4,475,059 of R. A. Sink, "Image Intensifier Tube with Reduced Veiling Glare and Method of Making the Same", is directed to forming a colored, low reflective light absorbing layer in the face plate of the tube adjacent any surface at which off-axis light could be reflected to the photoemissive device. Another approach to reducing veiling glare is that described in co-pending patent application, Ser. No. 655,399, Filed Sept. 27, 1984 now abandoned in favor of continuation application Ser. No. 899,768, filed Aug. 22, 1986 to M. J. Drinkwine, entitled "Image Intensifier Tube with Reduced Veiling Glare and Method of Making the Same", wherein a surface of the face plate is formed with a non-reflective conductive coating. The present invention takes a different approach to this problem.
It is, therefore, an object of the present invention to provide a light image intensifier tube with reduced veiling glare which is economical to make. It is a further object of this invention to provide a method of making such a tube in a highly economical and efficient manner.